the use of sapphire substrates in the production of semiconductor devices, such as silicon-on-sapphire (SOS) devices, has increased significantly in recent years. The term sapphire indicates alpha aluminum oxide (Al.sub.2 O.sub.3). It is necessary in the production of such devices that the sapphire substrates be marked with identification and alignment marks. The latter are particularly important for the aligning of patterned resist layers for processes such as localized oxygen implants and the like. It will be appreciated that, as the dimensions of the pattern to be formed in or on the sapphire substrate shrink, the necessity for accurate alignment of photomasks and the like becomes more acute.
One method of marking sapphire substrates which has been utilized is the engraving of a mark into the substrate with, e.g. a diamond tool. Although effective, engraving of sapphire substrates produces a stress which can cause cracking of the substrate as a result of the heating and cooling that are necessary to form a device thereon. Sapphire substrates are subject to cracking even when heating and cooling cycles are carefully controlled.
More recently, sapphire substrates have been marked with indicia, such as identification marks, by selective doping with an impurity which develops a color. Sasaki, U.S. Pat. No. 4,348,803, issued Sept. 14, 1982, discloses producing color indicia in a sapphire substrate by doping with chromium or titanium.
Saito et al. in two articles in the Japanese Journal of Applied Physics: Vol. 24, No. 8, pp. 1115 and 1116; and No. 11, pp. L880-L882, disclose coloration of quartz and sapphire, respectively, by metal ion implantation. While these processes are efficacious in that they color the substrate, they suffer from two principal disadvantages. First, as described in the articles, the color implants have a marked tendency to fade by varying degrees depending on the temperatures to which the substrates must be subjected. Second, the disclosed processes implant metals such as niobium, iron, copper, cobalt, titanium and chromium into the substrate. Such metals are a source of contamination which, potentially, could have an adverse effect on devices incorporating the substrate.
In accordance with this invention, a method has been found to provide indicia on a sapphire substrate which suffers from neither of the above disadvantages to a marked degree.